1. Field of the Invention
The present invention relates to a drive circuit for providing the excitation to an optical modulator of a modulator-integrate distributed-feedback laser diode (hereinafter simply referred to as an MI-DFB laser diode) for the use in a repeater or the like for optical communication.
2. Description of the Prior Art
With the requirements for long-distance transmission and larger capacity of an optical communication system, transmission characteristics of 100 Km or more and 2.5 Gb/s have been required. In order to meet such requirements, a modulator,integrate distributed-feedback laser diode in which a laser diode of a direct modulation system or an electro-absorption type optical modulator and a DFB laser diode are integrated has been researched and developed, and a circuit for driving the relevant light-emitting element is being developed in keeping with the above.
Here, a drive circuit for driving a laser diode of a direct modulation system according to a prior art of the present invention will be described. As shown in FIG. 1, this drive circuit is provided with an input buffer 1A for amplifying a signal required for direct modulation (hereinafter referred to as a modulation signal) and a differential amplifier 1B for receiving reference voltage VREF and a modulation signal SIN and outputting drive voltage Vm.
The differential amplifier 1B has a first to a fourth field effect transistors TNI to TN4 as shown in FIG. 2. In the first field effect transistor TN1, the drain thereof is connected to a ground line GND and the reference voltage VREF is supplied to the gate thereof. In the second field effect transistor TN2, the drain thereof is connected to one end of a laser diode 2, the source thereof is connected to the source of the first field effect transistor TN1, and the modulation signal SIN is supplied to the gate thereof. In the third field effect transistor TN3, the drain thereof is connected to respective sources of the first and second field effect transistors TN1 and TN2, the source thereof is connected to a power supply line VSS, and bias voltage VIP is supplied to the gate thereof.
In the fourth field effect transistor TN4, the drain thereof is connected to the drain of the second field effect transistor TN2, and the source thereof is connected to the power supply line VSS. The fourth field effect transistor TN4 as functions as a bias element for making the laser diode 2 emit light stably, and bias voltage VIB is supplied-to the gate thereof. The bias voltage VIB is supplied, since the device circuit current of the laser diode 2 differs due to manufacturing dispersion of the element, for the purpose of regulating the current. Besides, the device circuit from the laser diode 2, and this device circuit current is referred to as an oscillation threshold current in the laser diode 2 of a direct modulation system.
Next, the Operation of the relevant drive circuit will be described. First, when the modulation signal is amplified by the input buffer 1A and the amplified modulation signal SIN is outputted to the differential amplifier 1B, a drive current based on a signal of the difference between the reference voltage VREF and the modulation signal SIN passes to the laser diode 2 by the differential amplifier 1B operated based on the bias voltage VIP. For example, when the modulation signal SIN is at an "H" (high) level, the second field effect transistor TN2 is turned ON, and a drive current passes to the laser diode 2. With this, a laser light is outputted outside from the relevant element 2. When SIN is at an "L" (low) level on the contrary, the second field effect transistor TN2 is turned OFF. Hence, no drive current passes to the laser diode 2, but the laser light is not oscillated.
Now, according to a related art of the present invention, an MI-DFB laser diode of an external modulation system of less power consumption as compared with the laser diode 2 of a direct modulation system has been developed, and a circuit for driving this laser diode is being demanded.
The MI-DFB laser diode is described in M. Suzuki et al. "Monolithic Integration of InGaAsP/InP Distributed Feedback Laser and Electroabsorption Modulator by Vapor Phase Epitaxy: JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO.9, SEPTEMBER 1987", and is an element obtained by integrating an optical modulator and a DFB laser diode as a multi-giga-bit optical transmission system. The DFB laser diode is for generating a laser light, and the optical modulator is for controlling the external output of the laser light by absorbing or transmitting a laser light generated by the laser diode. Such MI-DFB laser diode is capable of driving at a high speed with a small-size, high output and low voltage, and the application thereof is being expected.
Next, a case when the drive circuit of the laser diode 2 of a direct modulation system is applied to the MI-DFB laser diode will be described. First, as shown in FIG. 3, one end of an outside resistance RL and an electrode (P side) for the optical modulator of the MI-DFB laser diode 3 are connected to the drain of the second field effect transistor TN2, respectively, and another end of the resistance RL is connected to a ground line GND. The electrode (P side) for a laser oscillation of the laser diode 3A is connected to a power supply line VCC through a constant current source 4, and the source of a third field effect transistor TN3 is connected to a power supply line VSS at approximately -5 V. The resistance RL is connected for the purpose of voltage-driving of the optical modulator 3B. Each electrode of N side thereof are connected to a ground line GND.
Here, the operation of this drive circuit such a shown in FIG. 3 will be described. First, when the modulation signal SIN shows an "H" (high) level in a state that laser diode 3A is oscillating, the second field effect transistor TN2 is turned ON, and a current I passes in a resistance RL. At this time drive voltage Vm becomes approximately -3 V, which is supplied to the optical modulator 3B as reveres bias voltage. A device circuit current Imod passes to the drive circuit from the optical modulator 3B at this time. The current I passing in the load resistance RL passes into the third field effect transistor TN3 through the second field effect transistor TN2, and the current Imod passes into the fourth field effect transistor TN4. With this, the laser light emitted from the inside of the laser diode 3A is absorbed by an electric field in the optical modulator 3B, and the output light to the outside is intercepted.
On the contrary, when the modulation signal SIN shows an "L" (low) level, the second field effect transistor TN2 is turned OFF, and the current I in the resistance RL becomes zero. At this time the drive voltage Vm also becomes 0 V, and the current Imod passes into the fourth field effect transistor TN4 from the optical modulator 3B. Since the reverse bias voltage is not applied to the optical modulator 3B at this time, the laser light emitted from the laser diode 3A passes through without being absorbed by the electric field in the optical modulator 3B and is outputted outside.
However, the currents Imod of the laser diode 2 and the MI-DFB laser diode 3 are different from each other as shown in FIG. 4. In FIG. 4, the axis of ordinates represents the current Imod, and the axis of abscissas represents the modulation signal SIN, respectively. A dashed line shows the current Imod of the laser diode 2 for the modulation signal SIN of the differential amplifier 1B, and a solid line shows the current Imod of the MI-DFB laser diode 3 for the signal SIN, respectively.
According to the foregoing, as against that the current Imod of the laser diode 2 of a direct modulation system is constant, the current Imod when the modulation signal SIN is at an "H" level and the current Imod when the modulation signal SIN is at an "L" level are different from each other in the case of the MI-DFB laser diode 3 of an external modulation system. Such a phenomenon has been confirmed by the present inventor et al., and is considered as wraparound of a DC current into the optical modulator 3B from the constant current source 4 for driving the laser diode 3A.
In a method that the drive circuit of the laser diode 2 is applied to the MI-DFB laser diode 3 as it is and the gate of the fourth field effect transistor TN4 is biased fixedly by the bias voltage VIB, there is such a problem that the regulation of the current Imod corresponding to the modulation signal="H" or "L" level becomes incomplete, and a noise is contained in an output waveform or the laser light becomes unstable due to the fact that the drive voltage (drive signal) Vm undergoes a change.
Thus, such a laser driver that is structured so as to sufficient for driving the optical modulator 3B of the laser diode 3 and is capable of controlling the optical output of the laser diode 3 with high precision is demanded.